1. Field of the Invention
The present invention relates to an apparatus and method for manufacturing optical fibers, and more particularly, to an apparatus and method for manufacturing optical fibers by a so-called modified chemical vapor deposition (MCVD) process in which reactant gases are injected into a heated external circular tube to generate particles therein to adhere onto the inner wall of the external circular tube, and another gas is jetted through several holes in the vicinity of the front end of the internal tube to enhance deposition efficiency and reduce tapered entry length.
2. Description of the Related Art
An optical fiber transmits a light signal and is comprised of a core having a high refractive index and an outer clad surrounding the core. Such an optical fiber can be generally produced by MCVD (modified CVD), OVD (outside vapor deposition) or VAD (vapor axial deposition). In particular, MCVD is most widely used in fabricating optical fibers since the mechanism of generation and deposition of particles are clearly understood and the reaction occurring inside a quartz tube enables to reduce contaminants. An example of MCVD is illustrated in FIG. 1.
Referring to FIG. 1, a circular tube 10 having a diameter of 10xcx9c30 mm, e.g., a quartz tube, is installed to be rotatable at a speed of 60xcx9c120 rpm. The circular tube 10 is heated at 1600xcx9c1800xc2x0 C. by a torch 11 moving in a direction along the axis of the circular tube 10. The torch 11 moves at a low speed of 10xcx9c30 cm/min. Here, mixed reactant gases such as SiCI4, GeCI4, POCI3, BCI3, O2, etc. are supplied through the entrance of the circular tube 10. These gases reach a reaction area (R) in a portion heated by the torch 11 to induce a chemical reaction, thereby creating fine particles, e.g., SiO2, GeO2, P2O5, BCI3, etc. Once a particle is formed at a given radial position within the tube 10, a particle trajectory results from the thermophoretic force generated in the radial direction of the circular tube 10 by the temperature field. Initially, the particles move inward since temperature of the inner wall of the tube 10 is hotter than the gas temperature. Farther downstream from the torch 11, the wall is cooler than the gas and particles move toward the tube wall. Thus, these particles are deposited on the wall, to then form a glass layer 12. Certain trajectories near the wall result in deposition, while particles near the center are swept out of the tube.
Subsequently, the glass layer 12 is heated by the moving torch 11, to be sintered into a glassy material. When the torch 11 reaches the end of the circular tube 10, it returns to its starting position and the above-described procedure is repeated, thereby allowing the glass layer 12 to have a multilayered structure. The composition of the each layer 12 is controlled by altering the relative concentration and species of reactant gases according to the particular objective of the fiber such as single-mode fiber, multi-mode fiber, erbium-doped fiber or special fibers.
If the glass layer 12 is formed to a desired thickness, introduction of reactant gases is terminated and the circular tube 10 is heated by the torch 11 to 2,000xc2x0 C. or higher. Then, the circular tube 10 shrinks into a rod shape due to surface tension to be the preform of an optical fiber. The preform is heated in a furnace, drawn to be a fine wire shape, and then coated, thereby completing the optical fiber.
However, in the MCVD process, since the axial velocity of particles near the center of the tube is relatively large compared to the thermophoretic velocity and concentration of the particles is high near the center, a large amount of particles are swept out of the tube, which results in a low particle deposition efficiency. Also, as shown in FIG. 1, the particles produced in the reaction area (R) travel along loci 13 indicated by solid lines in the drawing. That is, the traveling path of a particle until it adheres to the inner wall of the circular tube is long, which increases the distance between a point at which particles initially adhere to the circular tube and a point at which the thickness of the glass layer 12 becomes a constant level, which will be referred to as a tapered entry section (L). The tapered entry section (L) is not useful for making optical fibers and the portion corresponding to the tapered entry section (L) must be cut out.
In order to increase the particle deposition efficiency, particles are produced by installing a heating element inside the circular tube 10, as disclosed in Sinclair, et al. (U.S. Pat. No. 4,263,032) and Buehl (U.S. Pat. No. 4,328,017). However, since the particles produced by these methods are also deposited on the inner wall of the circular tube after they travel a considerable distance along the tubular axis, it is known that the aforementioned tapered entry section (L) cannot be reduced.
To overcome this drawback, a method of reducing the tapered entry section is taught in the Korean Patent Laid-open Gazette No. 96-41134 (Dec. 19, 1996). As shown in FIG. 2, an internal tube 20 serving as a heating element is inserted into an external tube 10 into which reactant gases are injected, and high-temperature gases are radially jetted from the peripheral surface in the vicinity of the front end 23 of the internal tube 20. In other words, a plurality of jet holes 22 leading to a bore 21 are radially formed on the peripheral surface in the vicinity of the front end 23 of the internal tube 20. Gases, e.g., O2, supplied through the bore 21 are radially jetted through the jet holes 22. The gas jets form a film around the internal tube 20, exerting the effect of enlarging the internal tube 20. Also, since the gas jets are in a high-temperature, a high temperature gradient can be achieved. Thus, thermophoretic velocity of particles are increased due to the increased temperature gradient. In addition, gas jetting through the jet holes 22 drives particles to the inner wall of the external tube. Consequently, the increased radial velocity of particles due to the gas jet increases deposition efficiency. Above all, since the flow loci of the particles generated from the reactant gases are reduced in length, a tapered entry section Lxe2x80x2 can be noticeably reduced, as shown in FIG. 2.
However, it is known that the above-described optical fiber fabrication method using an inner jet is not actually effective in view of particle deposition efficiency, compared to the conventional MCVD process. In detail, in the MCVD process using inner annular jets, the particles created in the reaction area (R) tend to collide with and stick onto the front end 23 of the internal tube 20 while moving toward the inner wall of the external tube 10. The collision of particles results in accumulation of the particles around the front end 23 and the amount of accumulated particles gradually increases. Finally, the accumulated particles obstruct the jet holes 22 of the internal tube 20, which may impede smooth jetting.
To solve the above problems, it is an objective of the present invention to provide an apparatus and method for manufacturing optical fibers having an improved particle deposition efficiency by providing a separate jet hole for supplying jets to the front end of an internal tube to prevent accumulation of reactant particles and to induce smooth flow of the reactant particles.
To achieve the above objective of the present invention, an apparatus is provided for manufacturing optical fibers. The apparatus includes an external circular tube into which reactant gases are introduced from an entrance at one end thereof. An internal circular tube is inserted into the external circular tube through an exit at the other end of the external circular tube. The internal circular tube has a bore to which gas jets are supplied. Jet holes are formed to lead to the bore on the outer circumferential surface in the vicinity of the front end of the internal circular tube. The jet holes are for radially jetting the gas jets. An auxiliary jet hole is formed at the front end of the internal circular tube to lead to the bore. The auxiliary jet hole is for jetting the gas jets frontward. A torch is installed to reciprocate along the axial direction of the external circular tube, for heating the external circular tube to induce a chemical reaction of the reactant gases to thus generate particles.
The optical fiber manufacturing apparatus may further include means for heating the internal circular tube.
According to another aspect of the present invention, there is provided a method for manufacturing optical fibers using an optical fiber manufacturing apparatus. The apparatus includes an external circular tube and an internal circular tube positioned within the external circular tube. The internal circular tube has jet holes and an auxiliary jet hole. The jet holes are formed on the circumferential surface to lead to a bore, in the vicinity of the front end thereof. The auxiliary jet hole is formed at the front end thereof. A torch is installed to reciprocate along the axial direction of the external circular tube. The method includes the steps of injecting reactant gases from the entrance at one end of the external circular tube; heating the external circular tube with the torch to thereby generate particles from the reactant gases; and supplying gas jets to the bore of the internal circular tube to radially jet the gas jets through the jet holes and to jet the same through the auxiliary jet hole in a direction opposite to the direction of flow of the reactant gases.
This method can also be used to manufacture optical fiber amplifier by injecting rare-earth-materials-doped gases to the bore of the internal circular tube or by placing rare earth materials inside the internal circular tube.